The present invention relates to an optical device and, more particularly, to a binocular device having a wide field-of-view which is capable of providing monochrome or multicolor images.
Miniaturization of electronic devices has always been a continuing objective in the field of electronics. Electronic devices are often equipped with some form of a display, which is visible to a user. As these devices reduce in size, there is an increase need for manufacturing compact displays, which are compatible with small size electronic devices. Besides having small dimensions, such displays should not sacrifice image quality, and be available at low cost. By definition the above characteristics are conflicting and many attempts have been made to provide some balanced solution.
An electronic display may provide a real image, the size of which is determined by the physical size of the display device, or a virtual image, the size of which may extend the dimensions of the display device.
A real image is defined as an image, projected on or displayed by a viewing surface positioned at the location of the image, and observed by an unaided human eye (to the extent that the viewer does not require corrective glasses). Examples of real image displays include a cathode ray tube (CRT), a liquid crystal display (LCD), an organic light emitting diode array (OLED), or any screen-projected displays. A real image could be viewed normally from a distance of about at least 25 cm, the minimal distance at which the human eye can utilize focus onto an object. Unless a person is long-sighted, he may not be able to view a sharp image at a closer distance.
Typically, desktop computer systems and workplace computing equipment utilize CRT display screens to display images for a user. The CRT displays are heavy, bulky and not easily miniaturized. For a laptop, a notebook, or a palm computer, flat-panel display is typically used. The flat-panel display may use LCD technology implemented as passive matrix or active matrix panel. The passive matrix LCD panel consists of a grid of horizontal and vertical wires. Each intersection of the grid constitutes a single pixel, and controls an LCD element. The LCD element either allows light through or blocks the light. The active matrix panel uses a transistor to control each pixel, and is more expensive.
An OLED flat panel display is an array of light emitting diodes, made of organic polymeric materials. Existing OLED flat panel displays are based on both passive and active configurations. Unlike the LCD display, which controls light transmission or reflection, an OLED display emits light, the intensity of which is controlled by the electrical bias applied thereto. Flat-panels are also used for miniature image display systems because of their compactness and energy efficiency compared to the CRT displays. Small size real image displays have a relatively small surface area on which to present a real image, thus have limited capability for providing sufficient information to the user. In other words, because of the limited resolution of the human eye, the amount of details resolved from a small size real image might be insufficient.
By contrast to a real image, a virtual image is defined as an image, which is not projected onto or emitted from a viewing surface, and no light ray connects the image and an observer. A virtual image can only be seen through an optic element, for example a typical virtual image can be obtained from an object placed in front of a converging lens, between the lens and its focal point. Light rays, which are reflected from an individual point on the object, diverge when passing through the lens, thus no two rays share two endpoints. An observer, viewing from the other side of the lens would perceive an image, which is located behind the object, hence enlarged. A virtual image of an object, positioned at the focal plane of a lens, is said to be projected to infinity. A virtual image display system, which includes a miniature display panel and a lens, can enable viewing of a small size, but high content display, from a distance much smaller than 25 cm. Such a display system can provide a viewing capability which is equivalent to a high content, large size real image display system, viewed from much larger distance.
Conventional virtual image displays are known to have many shortcomings. For example, such displays have suffered from being too heavy for comfortable use, as well as too large so as to be obtrusive, distracting and even disorienting. These defects stem from, inter alia, the incorporation of relatively large optics systems within the mounting structures, as well as physical designs which fail to adequately take into account important factors as size, shape, weight, etc.
Recently, holographic optical elements have been used in portable virtual image displays. Holographic optical elements serve as an imaging lens and a combiner where a two-dimensional, quasi-monochromatic display is imaged to infinity and reflected into the eye of an observer. A common problem to all types of holographic optical elements is their relatively high chromatic dispersion. This is a major drawback in applications where the light source is not purely monochromatic. Another drawback of some of these displays is the lack of coherence between the geometry of the image and the geometry of the holographic optical element, which causes aberrations in the image array that decrease the image quality.
New designs, which typically deal with a single holographic optical element, compensate for the geometric and chromatic aberrations by using non-spherical waves rather than simple spherical waves for recording; however, they do not overcome the chromatic dispersion problem. Moreover, with these designs, the overall optical systems are usually very complicated and difficult to manufacture. Furthermore, the field-of-view resulting from these designs is usually very small.
U.S. Pat. No. 4,711,512 to Upatnieks, the contents of which are hereby incorporated by reference, describes a diffractive planar optics head-up display configured to transmit collimated light wavefronts of an image, as well as to allow light rays coming through the aircraft windscreen to pass and be viewed by the pilot. The light wavefronts enter an elongated optical element located within the aircraft cockpit through a first diffractive element, are diffracted into total internal reflection within the optical element, and are diffracted out of the optical element by means of a second diffractive element into the direction of the pilot's eye while retaining the collimation. Upatnieks, however, does not teach how the display could transmit a wide field-of-view, or tackle a broad spectrum of wavelengths (for providing color images).
U.S. Pat. No. 5,966,223 to Friesem et al., the contents of which are hereby incorporated by reference describes a holographic optical device similar to that of Upatnieks, with the additional aspect that the first diffractive optical element acts further as the collimating element that collimates the waves emitted by each data point in a display source and corrects for field aberrations over the entire field-of-view. The field-of-view discussed is ±6°, and there is a further discussion of low chromatic sensitivity over wavelength shift of Δλc of ±2 nm around a center wavelength λc of 632.8 nm. However, the diffractive collimating element of Friesem et al. is known to narrow spectral response, and the low chromatic sensitivity at spectral range of ±2 nm becomes an unacceptable sensitivity at ±20 nm or ±70 nm.
U.S. Pat. No. 6,757,105 to Niv et al., the contents of which are hereby incorporated by reference, provides a diffractive optical element for optimizing a field-of-view for a multicolor spectrum. The optical element includes a light-transmissive substrate and a linear grating formed therein. Niv et al. teach how to select the pitch of the linear grating and the refraction index of the light-transmissive substrate so as to trap a light beam having a predetermined spectrum and characterized by a predetermined field of view to propagate within the light-transmissive substrate via total internal reflection. Niv et al. also disclose an optical device incorporating the aforementioned diffractive optical element for transmitting light in general and images in particular into the eye of the user.
The above prior art virtual image devices, however, provide a single optical channel, hence allowing the scene of interest to be viewed by one eye. It is recognized that the ability of any virtual image devices to transmit an image without distortions inherently depends on whether or not light rays emanating from all points of the image are successfully transmitted to the eye of the user in their original wavelength. Due to the single optical channel employed by presently known devices, the filed-of-view which can be achieved without distortions or loss of information is rather limited.
Although attempts have been made to develop binocular viewing systems [see, e.g., U.S. Pat. No. 4,805,988 to Dones and International Patent Application No. WO 01/95027 to Amitai], these systems are rather bulky, heavily rely on relative positioning and/or relative alignment and are generally less than fully satisfactory. For example, in many traditional binocular viewing systems, two different image paths are outputted to the eyes of the user, and it is necessary to substantially align the these optical paths with the user's eyes, such that the center of each image falls on the center of the user's field of view. Any departure from such alignment can have many undesirable consequences including loss of image quality, as perceived by the user, user eye strain or fatigue.
There is thus a widely recognized need for, and it would be highly advantageous to have wide field-of-view binocular device, devoid the above limitations.